This invention relates to a process for controlling the dihydride and monohydride bond densities in amorphous silicon hydrides produced by reactive rf sputtering of an amorphous silicon target.
Monohydride bonding is the configuration where only one of the four tetrahedral bonding sites of a silicon atom is bonded to a hydrogen atom. Dihydride bonding has two of the sites filled by hydrogen atoms. The interest in producing exclusively monohydride bonding or substantially pure silicon monohydride comes from the reported conductivity and better semiconductor properties of substantially pure silicon monohydride films.
Amorphous films combining silicon and hydrogen in various concentrations and bonding configurations have been produced by both glow-discharge deposition from silane and by reactive rf sputtering in a hydrogen-argon atmosphere. The glow-discharge process results in a combination of dihydride and monohydride bonding configurations, if deposited on a substrate maintained at a temperature below 200.degree. C., but exclusive monohydride bonding is obtained if the substrate is held at a temperature above 200.degree. C. The glow-discharge method has produced variations in the type of bonding in the amorphous film but has been limited in the range of hydrogen density it can produce.
Reactive sputtering has produced uncontrolled combinations of monohydride and dihydride bonding, but can be used to produce films of varying hydrogen density, principally through variation of the argon to hydrogen ratio; however, reactive sputtering has not been capable of producing films with an exclusive monohydride bond or of producing films in which the ratio of monohydride to dihydride bonding can be preselected.
M. H. Brodsky in a paper entitled "PLASMA PREPARATIONS OF AMORPHOUS SILICON FILMS" published in 1978 in Thin Solid Films, Volume 50, pages 57-67, principally sets out and reviews the two plasma methods for the preparation of hydrogenated amorphous silicon films, these two methods being the above-referenced silane glow discharge decomposition and the argon-hydrogen reactive sputtering. As stated by Brodsky in his paper, a reactive sputtering method for independently controlling the bonding sites, that is the ratio of monohydride and dihydride bonding, has not yet been reported.